A high electron mobility field effect transistor (referred to as HEMT hereinafter) has been used as an important component of a high-frequency communication instrument. A big feature of the HEMT is to have a selectively doped hetero structure comprising an electron supplying layer (a doped layer) for supplying electrons and a channel layer through which electrons run, these layers being made of different materials. In this hetero structure, electrons supplied from n-type impurities within the electron supplying layer are pooled in a potential well formed at a channel side of a heterojunction interface due to a difference of electron affinity between materials constituting the hetero junction, and then a two-dimensional electron gas is formed. Since n-type impurities supplying electrons are present in the electron supplying layer as described above and the electrons supplied from the layer run through a high purity channel so that the ionized impurities and the electrons are spatially separated from each other, the two-dimensional electron gas within the channel is hardly scattered by the ionized impurities, and consequently a high electron mobility is realized.
Although the HEMT has usually been fabricated by using an epitaxial substrate in which thin film crystal layers respectively having predetermined electronic characteristics are laminated and grown on a GaAs single crystal substrate so as to have a predetermined structure, it has been required to precisely control the thin film crystal layer forming the HEMT structure on the order of monoatomic layer level for the purpose of imparting a high electron mobility to the channel. Therefore, a molecular beam epitaxy (referred to as a MBE method, hereinafter) or a metalorganic chemical vapor deposition (referred to as a MOCVD method, hereinafter) has conventionally been used as a method for manufacturing an epitaxial substrate having a HEMT structure.
The MOCVD method, especially among other methods as described above for growing epitaxial substrates, uses an organometallic compound or a hydride of atomic species constituting an epitaxial layer as a source material and then pyrolyzes the source material on the substrate to grow a crystal thereof, so that this method is applicable to a wide range of substances and is not only suitable for precisely controlling the crystal composition and thickness thereof but also capable of processing a large amount of substrates with favorable controllability, and consequently this method has recently been used widely and commercially.
Although materials such as GaAs and AlGaAs have widely been used as III-V compound semiconductors for these epitaxial substrates since these materials with any compositions can match the lattice constants thereof with each other and allow for producing various hetero junctions while keeping good crystallinity thereof, it is necessary to further improve the electron mobility in the channel layer in order to enhance the performance of the HEMTs. Therefore, InGaAs has recently been used as a material for the channel layer instead of using GaAs, because InGaAs has extremely excellent properties as the III-V compound semiconductor used for the hetero junction, that is, InGaAs is not only excellent in its electron transporting characteristic but also capable of significantly changing its energy gap in accordance with the In composition and further capable of effectively confining the two-dimensional electrons. In addition, AlGaAs or GaAs is known as a material to be combined with InGaAs.
However, the InGaAs cannot be lattice-matched with GaAs, so that it has conventionally been impossible to obtain an epitaxial substrate having substantial physical properties by using a InGaAs layer. However, it has subsequently been found that a reliable hetero junction can be formed without unfavorably inducing a decrease in crystallinity such as producing a dislocation even when a material with lattice misfit is used provided that the misfit is within a limit of elastic deformation, so that there has practically been used an epitaxial substrate described above.
An epitaxial growth substrate having a structure in which the above described InGaAs layer is used as a channel layer part of the conventional HEMT through which two-dimensional electrons flow has been utilized for fabricating an electronic device which has a higher mobility and is excellent in a noise characteristic compared with the conventional device. The HEMT, in which InGaAs layer is used for the channel layer through which two-dimensional electrons flow, is referred to as a pseudomorphic high electron mobility transistor (hereinafter, referred to as a pHEMT).
A limit value of a thickness of the strain crystal layer in the above described lattice misfit material are given as a function of the strain crystal layer composition, and as for an InGaAs layer with respect to a GaAs layer for example, a Matthews' theoretical equation disclosed in J. Crystal Growth, 27 (1974) p. 118 and in J. Crystal Growth, 32 (1976) p. 265, is known, and this theoretical equation has been found to be experimentally correct as a whole.
JP-A-6-21106 discloses a technique for improving an electron mobility, in which an In composition of an InGaAs strain layer used as a channel layer of a pHEMT structure and a thickness of the InGaAs layer are optimized by a certain relational expression, provided that a limit value of a thickness of the InGaAs layer given by the theoretical equation is assumed to be an upper limit of the thickness range.
Since it is effective to additionally reduce the scattering of two-dimensional electrons caused by ionized impurities in order to improve the mobility, a spacer layer which has the same material and composition as an electron supplying layer and to which any impurities are not added may be inserted between the electron supplying layer and the channel layer. For example, Japanese Patent No. 2708863 discloses a structure for improving a two-dimensional electron gas concentration and an electron mobility, in which a spacer layer consisting of an AlGaAs layer and a GaAs layer is inserted between an InGaAs strain layer used as a channel layer and an n-AlGaAs electron supplying layer of a pHEMT structure and the growth condition is optimized.
When an InGaAs strain layer is used as a channel layer of the pHEMT structure through which electrons run, it is possible to improve an electron mobility at room temperature (300 K) compared with an epitaxial substrate of the HEMT structure in which a GaAs layer is used as a channel layer. However, the mobility at room temperature (300 K) as has been reported before is 8000 cm2/V·s at the maximum, and thus it has been difficult to achieve an electron mobility exceeding the above described value even in the case of a pHEMT structure epitaxial substrate in which an InGaAs strain layer is used as a channel layer.
If the structure disclosed in Japanese Patent No. 2708863 is adopted in order to increase the electron mobility in the pHEMT structure epitaxial substrate, the electron mobility is improved with increases in a thickness of the spacer layer, however, a concentration of the two-dimensional electron gas formed in the channel layer decreases because a distance between the electron supplying layer and the channel layer becomes larger, and thus leads to an undesirable outcome.
In order to improve the electron mobility and the two-dimensional electron gas concentration in the channel layer simultaneously, it is effective to increase an In composition of the channel layer and to increase the layer thickness. This is because the increase in the In composition of the channel layer leads to a decrease in an effective mass of electrons which travel through the channel layer for improving the electron mobility, and further to make a difference of conduction band energy between the electron supplying layer and the channel layer larger, and consequently the two-dimensional electron gas concentration can be increased. In addition, it can be considered that the increase in the channel layer thickness may lead to a decrease in energy at an excited level of the two-dimensional electron gas, and thus may be effective for improving the two-dimensional electron gas concentration.
However, it is difficult to increase the In composition of the InGaAs strain layer and the thickness of the InGaAs layer while keeping a favorable crystal property of the InGaAs layer, because dislocation defects may be developed due to the lattice misfit with the GaAs layer. Further, in any of the above described prior arts, values of the two-dimensional electron gas concentration and the electron mobility in the pHEMT structure epitaxial substrate are not yet satisfactory in view of possibility that the characteristics of electronic devices become more favorable with the increase in these values.
Therefore, in the pHEMT structure epitaxial substrate which uses n-AlGaAs as the electron supplying layer and an InGaAs layer as the channel layer, it has strongly been desired to realize an epitaxial substrate having a higher two-dimensional electron gas concentration and a higher electron mobility compared with the currently reported values.
It is well known that the electron mobility is an important parameter for improving various characteristics such as an on-resistance, a maximum current value, or a transconductance each of which is an important performance indicator of a field effect transistor. Further improvement of the electron mobility can achieve a reduction in the build-up resistance (on-resistance). This leads to a reduction in power consumption, so that an operating time of the battery can be prolonged. At the same time, since a calorific value can be decreased, it is possible to realize higher integration of a device, and is also possible to increase a degree of freedom of modular design by reducing a chip size. From this viewpoint, it is desired to further improve the electron mobility in the case of a pHEMT which is used for various portable instruments such as a cell phone.